Estimate of oil persisting on the beaches of Prince William Sound 12 years after the Exxon Valdez oil spill.

We estimated the amount of oil remaining in Prince William Sound, Alaska, 12 yr after the 1989 Exxon Valdez spill to assess its importance as a long-term reservoir of toxic hydrocarbons. We found oil on 78 of 91 beaches randomly selected according to their oiling history. Surface oiling was recorded for randomly placed quadrats, which were then excavated and examined for subsurface oil. The cumulative area of beach contaminated by surface or subsurface oil was estimated at 11.3 ha. Surface oil varied little with tide height, but subsurface oil was more prevalent at the middle tide heights. The mass of remaining subsurface oil is conservatively estimated at 55 600 kg. Analysis of terpanes indicated that over 90% of the surface oil and all of the subsurface oil was from the Exxon Valdez and that Monterey Formation oil deposited after the 1964 Alaska earthquake accounted for the remaining surface oil. These results indicate that oil from the Exxon Valdez remains by far the largest reservoir of biologically available polycyclic aromatic hydrocarbons on beaches impacted by the spill and that biota dependent on these beaches risk continued exposure.